The present invention relates to a novel technique for disposing of waste water streams produced by acrylonitrile-manufacturing plants. More particularly, the present invention relates to a novel technique for treating waste water streams produced in acrylonitrile-manufacturing plants so as to recover some of the valuable components therein and simultaneously make the waste water sufficiently free of ecology-damaging components so that it can be discharged to waste without damage to the environment.
In the manufacture of acrylonitrile by the single step vapor phase ammoxidation of propylene with ammonia and oxygen, a waste water stream containing various organic and inorganic components is produced (hereinafter referred to as "acrylonitrile waste water"). Typically, acrylonitrile waste water contains about 3% organics and about 97% aqueous phase on a solids-free basis. The aqueous phase is composed predominantly of water and contains 2.4% ammonium sulfate and ammonium bisulfate. A typical composition of the organics phase of acrylonitrile waste water is as follows:
______________________________________ Component Weight % ______________________________________ HCN 9.6 methanol 0.1 acetaldehyde 2.1 acetonitrile 5.6 acrolein 2.9 acrylontrile 14.6 acetic acid 2.7 propionitrile 0.8 acrylic acid 47.7 pyrazine, pyridine 3.0 trans-fumaronitrile 3.1 acrylamide 3.3 methyl pyridine 2.1 succinonitrile 0.8 cyanopyridine 1.7 ______________________________________
The composition of acrylonitrile waste water may vary somewhat from the foregoing. However, in general acrylonitrile waste water will always be composed predominantly of water and contain at least ammonium sulfate, acrylic acid and acrylonitrile. A number of techniques have already been adopted for disposing of this waste water stream. In some operations, the waste water is simply discharged as is into extremely deep wells. In other operations, the waste water is incinerated.
Neither of these waste water disposal techniques is satisfactory from a long-term standpoint. Extremely deep wells are expensive to drill, and moreover continued discharge of acrylonitrile waste water into such wells is ecologically unsatisfactory. Incineration on the other hand is also disadvantageous because the sulfates in the waste water are transformed into sulfur dioxide which itself presents a significant air pollution problem.
In order to deal with the problem of discharging waste water from an acrylonitrile-manufacturing plant, it has been proposed to distill the waste water in order to significantly reduce the volume of the material which must be discharged. Unfortunately, the organic components, especially the acrylonitrile and acrylic acid, in the waste water causes fouling of the apparatus used for distilling, and hence distillation is not a practical answer.
It has also been proposed to purify the waste water with activated carbon thereby yielding a waste water stream substantially free of organic components. This partially purified waste water stream can then be distilled without the fouling problem mentioned above, if desired. Unfortunately, periodic regeneration of the activated carbon by conventional thermal treatment causes a five to ten percent loss in the total amount of activated carbon used for each regeneration step. In view of the expense of activated carbon and the relatively large amounts of activated carbon necessary, the use of activated carbon in this matter is economically prohibitive.
Accordingly, it is an object of the present invention to provide a novel technique for treating waste water produced in an acrylonitrile-manufacturing plant so that it can be discharged to waste without significant adverse impact on the ecology.
It is a further object of the present invention to provide a method for treating the waste water produced in an acrylonitrile-manufacturing plant so as to recover some of the valuable components therein prior to discharge of the waste water so that the expense of the waste water treatment process can be reduced.